Strain potential energy and efficiency question

[Dongorgon]

Homework Statement

A sphere of mass 0.1 kg is pushed against a rubber band catapult with a force of 80N, and released upwards.
Calculate:
A) The strain potential energy when stretched by 0.3m

B) The work done by the applied force

C) The speed of the sphere when released

D) The efficiency of the catapult:

The Attempt at a Solution

A)
E=1/2(80)(0.3)=12J

B)
WD=12J

C)
Considering the conservation of momentum:

1/2mv^2+(0.1)(0.3)g=12

Hence, |v|=15.3 m/s

D)
Truly unsure what to consider here or how to calculate this part?
I'm also unsure about the answer to C. Any help would be greatly appreciated here.
Thanks

 

[haruspex]

1/2mv^2+(0.1)(0.3)g=12

You've no basis for the gravitational PE term. The rubber was stretched by .3m, but that doesn't mean the sphere descended .3m in the process. If the catapult is very wide, it could have descended a lot more; at the other extreme, it may only have descended .15m.
Wrt part D, I've no idea either. You had to assume it's 100% efficient to answer part C. Are you sure there's no other info?